"For this we declare to you by a word from the Lord, that we who are alive, who are left until the coming of the Lord, will not precede those who have fallen asleep. For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel and the sound of the trumpet of God, and the dead in Christ will rise first."

The context and timing for this even is highly disputed among followers and theologians. Generally, the timing is considered to be unknown, because of a passage in Matthew 24:36:

"But concerning that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only."

The Rapture is related to Armageddon. Different doctrinal groups of Christianity have different takes on the events, but generally, the Rapture, the Tribulation, the Millennium, and the final battle (Armageddon) will happen in one sequence, or another. Some Christians go so far as to believe that the Rapture can by achieved by purposely triggering Armageddon, or as we humans would call it, World War III. This often takes the form of instigating violence between the nation of Israel and the rest of the Muslim world.